A rotatable anode X-ray tube usually comprises an envelope which surrounds a rotatably mounted anode disc that acts as a target and has a marginal portion called a focal track. Generally, this track is made of a relatively high atomic number material such as tungsten or molybdenum which readily emits X-rays when bombarded by high energy electrons. A source of electrons is disposed to direct the high energy beam on to the focal track and thereby generate X-rays which emanate therefrom. The focal track portion of the anode disc is generally disposed at a predetermined target angle with respect to the plane of the disc so that the focal spot area is inclined toward a radially aligned X-ray transparent window in the envelope. Thus the X-rays pass in a beam through the window and appear to be emanating from a radial projection of the focal spot area in the tube.
A substantial portion of the electron beam energy that strikes the focal track area is converted to heat which is manifested by a sharp increase in temperature of the target material, frequently as high as 3000.degree. C. In order to avoid pitting or otherwise damaging the focal track surface, the anode disc is rotated at high angular velocities, frequently in the order of 10,000 to 20,000 RPM for example, to move successive segments of the focal track rapidly through the focal spot area that is aligned with the electron beam. As the focal track and target disc rotate, the particular areas which are not being struck with the electrons from the cathode are given an opportunity to cool through radiant dissipation of the heat. Though some heat is dissipated through radiant energy, the heat build up in the disc is frequently greater than the amounts which are dissipated and when the electron beam continues to impinge upon the same track in subsequent rotations of the target disc, the material will become over-heated and possibly permanently damaged. Also, if the tube is allowed to over-heat, the bearings on the shaft which support the disc within the envelope can become inoperative.
Most rotating anode target structures which have been disclosed to the prior art included a beveled edge with the cathode beam impinging upon the beveled surface which in turn generated the X-rays and directs them through the window. Frequently, the beveled portion of the target anode is a layered bimetallic construction which readily conducts heat from the focal track area into the body of the anode. Such structures are not wholly satisfactory and do not allow maximum loads of cathode ray bombardment over a reasonably long period of time without causing over-heating.
Rotating anode X-ray tubes in which the electrons impinge upon changing surfaces are known to the art. U.S. Pat. No. 3,836,805 to Kok discloses an anode that is carried on a rotor driven by a stator and including a pinion gear movable on a slide so that the electron spot changes along the axis of the anode disc as the anode is rotated. The anode is axially shifted in response to a heat sensing device which drives the pinion gear and changes the positioning of the focal track. With the disclosed construction, the anode can produce substantial amounts of off-focus radiation which especially occurs because the cathode rays are directed at right angles to the surface of the anode, unlike the proposed construction.
The U.S. Pat. No. 2,926,270 to Zunick discloses a disc shaped, bevel-type anode which is rotated and wobbled to alter the track upon which the electrons impinge. Upon continual use, this relatively heavy anode can misalign easily from the desired axial setting of the tube and produce off-focus radiation.